The long-term goal is to carry out research on hematopoietic stem cell (HSC) development and maintenance. The immediate goal is to assess BMP4 function in regulating HSCs by studying a tissue-specific BMP4 hypomorphic mouse that has significantly fewer HSCs than wild type. I hypothesize that reduced BMP4 signaling in the long bones either directly alters HSCs, or indirectly alters HSCs, by impacting their niche. The specific aims of this proposal are: 1. To determine if the decreased number of HSCs in BMP4 hypomorphs is due to intrinsic or extrinsic defects. The functional activity of HSCs from BMP4 hypomorphs will be tested using both in vitro HSC/progenitor assays and by asking whether HSCs from mutants can rescue hematopoiesis when transplanted into lethally irradiated wild type recipients. Conversely, the ability of the mutant HSC niche to support WT HSCs following transplantation will be assayed. 2. To evaluate whether attenuated BMP4 signaling occurs in HSCs or their niche. To pinpoint which cell types participate in BMP4-mediated HSC pool regulation, the expression levels of BMP4 and downstream effectors of BMP4 signaling will be evaluated in HSCs as well as the HSC niche in WT and mutant mice. 3. To determine the mechanism by which BMP4 supports the HSC pool. The fraction of HSCs from BMP4 hypomorphs that are actively dividing, quiescent and undergoing apoptosis will be evaluated using flow cytometry. The proposed studies will significantly enhance our current understanding of how HSC number is normally maintained in adults. Project Narrative: Blood stem cells have tremendous therapeutic potential. Identifying the molecules that normally control their survival and proliferation is critical for developing methods to control blood stem cell expansion for medical therapies that treat diseases such as bone marrow failure. The research goals are to determine how the signaling factor BMP4 functions to support the proliferation and maintenance of stem cells in their normal environment.